MacConkey agar results

Andre Burnens BURNENS at VBI.unibe.ch
Thu Apr 11 11:33:35 EST 1996

Dear ,ory

Depending on fermentation activity, the colonies get very different hues 
of red on MacConkey. Very strongly acid-producing strains will give 
precipitations of bile acids in the agar surrounding the colonies in 
addition to being bright red. The amount of acid produced will strongly 
depend on the type of sugars used, the metabolic pathways for their 
degradation, and the type of fermentation end products observed. Finally, 
weak acid producers may revert the color of the agar because the alcaline 
byproducts of the aerobic metabolism of the peptone present in the medium 
will not only neutralize the acid produced in the beginning, but change 
the medium pH sufficiently to make the neutral red indicator in the 
medium turn yellow.

Hope this is of use to you, Andre

Andre P. Burnens MD
Institute for Veterinary Bacteriology, University of Berne
Laenggass-Strasse 122, CH-3012 Berne, Switzerland
Tel. +41 31 631 24 85       Fax. +41 31 631 26 34

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